Optical inspection method and apparatus

ABSTRACT

Apparatus for optical inspection of particulate material such as diamond abrasive comprises a body (1), an open sample container (25) which can rise and fall in the body, a reference member (71) above the container, mounted in a removable compaction head (9) of which the height relative to the body is fixed, and an optical inspection head (11) mountable on the body in place of composition head. The container is raised into contact with the reference member so that the surface of the sample is levelled and placed at a predetermined height. The compaction head is then removed and replaced by the optical inspection head. Consequently, the sample surface is always presented to the optical inspection head in a predetermined relationship and in particular at a predetermined height.

FIELD OF THE INVENTION

This invention relates to the optical inspection of samples ofparticulate material and in particular, the the preparation andpresentation of such samples for optical inspection.

BACKGROUND OF THE INVENTION

Optical inspection can be used to determine properties of particulatematerial, for example colour and reflectivity. One particularapplication of optical quality measurement is in monitoring the cosmeticquality of synthetic diamond crystals that are to be used as abrasivematerial. In one known measurement system, a sample is placed in a pot,that is to say an open container, and the exposed surface of thematerial is inspected by a suitable optical measurement device. Usually,the sample surface is uneven, and its height relative to the measurementdevice is inconstant, depending on the size of the particles. In thecase of saw diamond abrasive, even though each sample is taken from aspecific size range, there is still a variation of size and shape ofparticles within the sample. The resulting uneven sample surface andvariations in surface height relative to the measurement devicematerially reduce the accuracy and repeatability of the measurementresults, and measurement on one sample may not be readily comparablewith measurements on other samples.

SUMMARY OF THE INVENTION

An object of the present invention is to improve the repeatability andaccuracy of optical inspection measurements on particulate material, andin particular to provide for improved presentation of samples foroptical inspection.

According to one aspect of the invention there is provided a method ofpreparing a sample of particulate material comprising placing the saidmaterial in an open container, providing a reference surface at apredetermined position, moving the container thereby bringing an exposedsurface of the particulate material therein into intimate contact withthe reference surface, holding the container in the position thusdefined, and removing the reference surface, thereby presenting saidsample with an exposed surface which is substantially even and locatedat a predetermined position defined by said reference surface.

According to another aspect of the invention there is provided apparatusfor the optical inspection of particulate material, comprising asupport, an open sample container mounted for movement in heightrelative to the support, a reference member having an undersurfaceplaced at a predetermined height relative to the support and such thatthe sample of particulate material previously placed in the containercan be raised into contact of the said undersurface, and means forholding the container in the position defined by such contact, thereference member being movable away from the container at said positionto permit subsequent optical inspection of the exposed surface of thesample at the said position.

According to yet another aspect of the invention there is provided anoptical inspection apparatus for particulate material, comprising abody, a container movably supported in the body for containing a sampleof said material, a reference member having a reference and compactionsurface on its underside and locatable above the container with the saidsurface at a predetermined position, and means for moving the containercontaining a said sample towards the said member when the latter is solocated, thereby urging a said sample against the said surface, andmeans for holding the container in the thus-defined position when thesaid member is removed, so as to leave a compacted and even samplesurface exposed for optical inspection.

The reference member or reference surface is preferably used not only todefine the position of the sample surface but also to compact the sampleand level the sample surface. Accordingly the sample will normally beurged against the reference member under a certain amount of pressure.

In a preferred arrangement, respective carriers for optical inspectiondevices and for the reference member are interchangeably mountable onthe body or support, the reference member being removed after the samplehas been set in position, and being replaced by the optical devices,such that the position of the optical devices relative to the exposedsample surface is always a predetermined relation defined by theposition of the reference member, and in particular, the height of theoptical devices relative to the sample surface is always constant.

The present invention provides that the sample surface is alwaypresented at a consistent height, and further, that the sample surfaceis rendered even. Consequently, the accuracy and repeatability of theoptical measurements are greatly improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described with reference to the drawings,in which:

FIG. 1 shows the apparatus in section on a vertical diametral plane 1--1of FIG. 2, with a sample-preparation or compaction head in position,

FIG. 2 is a section on the plane 2--2 of FIG. 1, and

FIG. 3 shows an optical measurement head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The illustrated apparatus comprises a cylindrical body 1, within thelower region of which is a coaxial vertical bore 3 opening into acoaxial chamber 5 in the upper region of the body. The chamber 5 is openat the top.

The upper end of the body has a peripheral rebate 7. On this upper endcan be located a removable compaction head 9 as shown in FIG. 1, or aremovable measurement head 11 as shown in FIG. 3.

Each of these heads comprises a circular cover or support plate, 13, 15,with a depending annular flange 17 which fits closely on the rebate 7.Seated in the annular uppermost surface of the body are three balls 19placed at equal angular intervals about the vertical central axis of thebody, forming a three-point seating for the cover plate 13 or 15. Theseballs, together with the interaction between the flanges 17 and therebate 7, ensure that the compaction head, and the measurement head, arealways seated on the body in precisely the same position, and inparticular at the same height.

Each of these heads has a central aperture. Mounted in the centralaperture of the measurement head 11 is a measurement unit comprising aring of infra-red LEDs 21 each set at 45° to the central axis, and adetector 23 on the central axis, for optical inspection of a sampleplaced in a sample pot 25 in the chamber 5.

Infra red illumination is used for the reflectivity measurements owingto the fact that the samples may vary in colour. This colour should beneglected when making the measurement--by using the IR region of thespectrum all colours appear `grey`. Thus there can be changes or mixedcolours within the sample; this will not affect the measurement beingmade.

The precise nature of the optical measurement equipment provided willdepend on the nature of the measurement required. For example, if colourchanges are to be monitored, the light emitting devices may emit visiblelight. Suitable optical measurement equipment is well known andtherefore this part of the apparatus will not be further described.

The means provided in the base for supporting the sample pot 25, and thecompaction head 9, are designed to ensure that samples in the pot 25 arepresented to the measurement head as consistently as possible, and inparticular to ensure that the constituent particles in the sample arelevelled and their exposed upper surface is always presented at the sameheight.

In the bore 3 is mounted a guide sleeve 27, within which is a verticallyslidable plunger 29, urged upwards by a compression spring 31 seated inthe base of the body.

At the upper end of the plunger 29 is a horizontal support plate 33 withan upstanding concentric annular flange 35 which forms a locating seatfor the sample pot 25. The flange 35 has an in-turned lip with angularlyspaced interruptions, and the base of the sample pot has correspondinglyspaced lugs 37 which can be inserted through the interruptions in thelip and then engaged below the lip by rotation of the sample pot, in themanner of a bayonet connection. Seated in the upper surface of the plate33 are three support balls 39 at equal angular intervals, which hold thelugs 37 in tight contact with the underside of the flange lip, therebyensuring that the sample pot 25 cannot move vertically relative to theplunger 29.

The periphery of the plate 33 is connected to a shoulder in the base ofthe chamber 5 by an annular rubber sealing gasket 41, so that the upperregion of the chamber 5 is sealed from its lower region and from thebore 3 and plunger 29, so that the plunger and associated componentscannot be fouled by sample particles which may be spilled in the chamber5.

A generally horizontal lever 43 with a handle 45 extends through ahorizontal passage 47 in the body and is connected to the upper regionof the plunger 29 by a horizontal pivot pin 49.

Adjacent the bore 3, the upper side of the lever rests against ahorizontal fulcrum 51 in the upper region of the passage 47. On theopposite side of this fulcrum from the pivot pin 49, the lever rests onor is coupled to the piston rod 53 of a pneumatic damper cylinder ordashpot 55 mounted in the lower region of the body 1 with its axisvertical.

It will be seen that as the spring 31 urges the plunger 29 upwards, theouter region of the lever will pivot downwards about the fulcrum 51,pushing the piston rod 53 into the pneumatic cylinder 55, whichaccordingly damps the upward movement of the plunger 20 under the actionof the spring. To lower the plunger 29, the handle 45 is raisedmanually. The cylinder 55 does not damp this movement.

The lever 43 can be locked in its raised position (with the plunger 29lowered) by means of a spring-loaded locking pin 57 slidable in a borein the body 1, at right angles to and intersecting the passage 47 in thelower region of the latter. When the lever 45 is fully raised, the pincan slide across the passage 47 below the lever, thereby locking thelever in its raised position and the plunger 29 in its lowered orretracted position.

Coaxial with the locking pin 57 but on the opposite side of the passage47 is a locking screw 59 screw-threaded in the body 1 and provided withan operating knob 61. With the plunger 29 raised and the lever 43correspondingly lowered, the locking screw can be tightened against theside of the lever to lock it in any selected position, thereby locatingthe plunger at any selected height.

The compaction head 9 comprises a boss 63 in which is journaled acentral vertical rotatable shank 65 projecting downwards so as to belocated in the chamber 5 when the measurement head is seated on thebody 1. At the upper end of the shank 65 is a hand wheel or knob 67 bywhich the shank can be rotated about its vertical axis. At the lower endof the shank is a transverse horizontal support plate 69, on theunderside of which is removably fastened a carbide disc 71 with a planehorizontal lower surface. The shank and carbide disc are mounted in sucha way that the lower surface of the carbide disc is always at a fixedpredetermined distance below the lower surface of the cover plate 13 ofthe compaction head, and therefore at a fixed predetermined height inrelation to the body 1. The mounting may provide for initial adjustmentor subsequent correction of this vertical spacing, but in operation itis kept constant.

The carbide disc is coaxial with and fits closely within the sample pot25.

The operation of the apparatus will be described in relation to theinspection of the cosmetic quality of synthetic diamond crystals, inparticular saw diamond abrasive, with reference to the surface conditionand internal characteristics such as the presence of metallicinclusions. Crystals that are clear or contain only slight metallicinclusions are graded as "high/top quality", those with a large numberof inclusions are graded "low quality". The apparatus is required togive a numerical output grading the batch being examined according toquality.

Typically, saw diamond abrasive comprises particles in the size range200 μm to 2 mm, with each batch containing a single size range. A sampleis taken, for example of approximately 200 carats, and is placed in thesample pot 25. The sample pot is locked onto the support plate 33 withthe latter held in its lowest position by means of the locking pin 57.

The compaction head 9 is then fitted on the base 1 and the locking pin57 is released, permitting the spring 31 to raise the plunger 29 andwith it the sample pot 25, against the damping action of the pneumaticcylinder 55.

The raising of the sample pot 25 brings the particle sample in it intocontact with the underside of the carbide disc 71, compacting theparticle sample against the carbide disc. It will be understood that, ifnecessary, means may be provided for holding the compaction head 9 downon the body 1, against the upward pressure exerted by the spring 31.

The pressure exerted by the spring 31 will compact the sample in the pot25 and level the surface of the sample against the carbide disc. Thelocking screw is then tightened against the lever 43, to lock the leverand thereby the plunger 29 and sample pot 25 in position with the samplesurface at a reference level defined by the underside of the carbidedisc 71.

To ensure an even surface of the sample, the inner base surface 73 ofthe sample pot is not flat but is profiled, for example with a shallowconical profile as shown in FIG. 1. The shape of this base surface isdetermined experimentally to achieve the pressure profile within thesample to ensur upper surface of the sample.

Additionally, the carbide disc can be rotated by knob 67 to improve thecompaction and levelling of sample.

It will be understood that the most appropriate profile of the samplepot bottom surface will to some extent depend on the size and nature ofthe particles being compacted.

With the sample pot thus locked in position and presenting the samplesurface at the reference level defined by the carbide disc 71, thecompaction head 9 is removed and a measurement head 11 is placed on thebase 1. The emitters 21 and detector 23 then measure the surfacereflectivity of the sample. This measurement may be recorded by acomputer and compared with previously known calibration limits set onthe machine. The measured reading can then be presented numericallyand/or can be classified as indicating high or low quality material.

Because the surface of the sample, irrespective of particle size andsample size, is always presented at the same position in space withrespect to the measurement devices, reliably repeatable measurement ispossible.

To calibrate the unit and ensure that the dynamic measurement range ofthe system is sufficient to resolve the differences within the samplesbeing examined the following procedure is adopted:

The sample pot is removed and the holder locked in its down position viathe locking pin. A calibration disc of height such that when placed inthe locked down position its surface would coincide with the position ofthe reference carbide disc, is placed in the machine. This disc ispainted white so that the signal obtained from the measurement unit setsthe top boundary for the high quality material. A similar black disc isused to define the lower boundary of the measurement. Hence the dynamicrange is set.

If measurements are very close together the white and black discs arereplaced by a second set with different reflectivities to redefine theMax and Min of the measurement range and thus stretch the separation ofthe closely contested samples.

While the invention has been described with particular reference todetermining the cosmetic qualities of saw diamond abrasive, theinvention is more generally applicable to other situations whereparticulate material is to be sampled and optically inspected ormeasured, for example in measuring the colour of other types ofsynthetic and natural diamond abrasives, cosmetic powders, and othermaterials.

I claim:
 1. A method of preparing a sample of particulate material comprising the steps of placing said material in an open container, providing a reference surface at a predetermined position, moving said container thereby bringing an exposed surface of said particulate material therein into intimate contact with said reference surface, holding said container in the position thus defined, and removing said reference surface, thereby presenting said sample with an exposed surface which is substantially even and located at a predetermined position defined by said reference surface and optically inspecting said exposed surface of said particulate material disposed at said position by optical inspection means disposed in predetermined manner relative to said position.
 2. The method as claimed in claim 1, wherein said particulate material is urged against said reference surface thereby compacting said particulate material.
 3. Apparatus for the optical inspection of particulate material, comprising a support, an open sample container mounted for movement in height relative to said support, a reference member having an undersurface placed at a predetermined height relative to the support and such that a sample of particulate material previously placed in said container can be raised into contact with said undersurface, and means for holding said container in a position defined by such contact said reference member being moveable away from said container at said position to permit subsequent optical inspection of an exposed surface of the sample at said position.
 4. Apparatus as claimed in claim 3, further including means for urging said container towards said reference member, for compacting said sample against said undersurface.
 5. Apparatus as claimed in claim 4 in which said container has an internal bottom surface profiled to provide between it and said undersurface a pressure distribution adapted for levelling the upper surface of said sample.
 6. Apparatus as claimed in claim 3, 4 or 5 in which the reference member is rotatable about a vertical axis for levelling the surface of the sample.
 7. Apparatus as claimed in claim 6 further including optical inspection means disposed or disposable at a predetermined height relative to said position for inspecting said sample surface after removal of said reference number.
 8. Apparatus as claimed in claim 3, 4, or 5 further including optical inspection means disposed or disposable at a predetermined height relative to said position for inspecting the sample surface after removal of said reference member.
 9. Apparatus as claimed in claim 8 in which said reference member is mounted on a carrier which is removable from said support, said carrier and support being provided with cooperating locating means such that when the carrier is located on the support the reference member has its undersurface at a predetermined level, said apparatus further including a second carrier similarly disposable removably on said support and carrying optical inspection means arranged to inspect the sample surface when the second carrier is disposed on said support, whereby the sample surface is always presented to said optical inspection means in a predetermined relationship determined by said reference member.
 10. An optical inspection apparatus for particulate material, comprising a body, a container movably supported in said body for containing a sample of said material, a reference member having a reference and compaction surface on its underside and locatable above said container with said surface at a predetermined position, and means for moving said container containing said sample towards said member when the latter is so located, thereby urging said sample against said reference surface, and means for holding said container in the thus-defined position when said member is removed, so as to leave a compacted and even sample surface exposed for optical inspection.
 11. An optical inspection apparatus as claimed in claim 10, wherein said container is removably supported on a platform which has its lower portion in the form of a plunger slidably received in a vertical hole in said body.
 12. An optical inspection apparatus as claimed in claim 11, wherein said platform is urged upwards by a spring acting between said plunger and a part of said body.
 13. An optical inspection apparatus as claimed in claim 12, wherein the upward motion of the platform is subject to mechanical damping.
 14. An optical inspection apparatus as claimed in any one of claims 10 to 13, wherein said container has a circular opening and a lower portion of said member providing said surface is circular and is arranged to close said container when said container is moved towards said member.
 15. An optical inspection apparatus as claimed in claim 14, wherein said lower portion of said member providing said surface is rotatable about an axis perpendicular to said surface.
 16. An optical inspection apparatus as claimed in claim 14 wherein the base of said container is shaped to provide an appropriate pressure profile within said sample when the latter is compacted so as to ensure an even sample surface.
 17. An optical inspection apparatus as claimed in any one of claims 10, 11, 12 or 13, wherein the base of said container is shaped to provide an appropriate pressure profile within said sample when the latter is compacted so as to ensure an even sample surface. 